1. Field of the Invention
The present invention relates to a radiation image detecting device for detecting a radiographic image through a grid, a radiation imaging system and an operation method thereof.
2. Description Related to the Prior Art
In a medical field, a radiation imaging system, for example, an X-ray imaging system using X-rays is known. The X-ray imaging system is constituted of an X-ray generating apparatus for producing the X-rays, and an X-ray imaging apparatus for taking an X-ray image formed by the X-rays passed through an object (a patient). The X-ray generating apparatus has an X-ray source for emitting the X-rays to the object, a source control device for controlling the operation of the X-ray source, and an emission switch for commanding the source control device to start X-ray emission. The X-ray imaging apparatus has an X-ray image detecting device for detecting the X-ray image by converting the X-rays passed through each part of the object into an electric signal, and a console that controls the operation of the X-ray image detecting device and saves and displays the X-ray image.
The X-ray image detecting device includes an image detector for converting the X-ray image into the electric signal, a controller for controlling the image detector, and the like. As the image detector, a flat panel detector (FPD) having a lot of pixels arrayed in two dimensions in an imaging area is widely used. Each pixel accumulates electric charge in accordance with an X-ray dose (a time-integrated X-ray value). After imaging, the electric charge accumulated in each pixel is read out to a signal processing circuit through a switching element such as a TFT (thin film transistor). The signal processing circuit converts the electric charge of each pixel into a voltage signal, and outputs the voltage signals as an X-ray image signal.
There is known an X-ray image detecting device that has an X-ray dose measurement function and an automatic exposure control (AEC) function (for example, Japanese Patent Laid-Open Publication No. 07-201490). In this X-ray image detecting device, one or a plurality of measurement pixels for measuring an X-ray dose is disposed in the imaging area of the image detector, together with normal pixels (X-ray image detection pixels) for detecting an X-ray image. This measurement pixel is used as a dose measurement sensor for measuring the X-ray dose. A measurement signal is read out of the measurement pixel at regular time intervals and integrated to measure the X-ray dose. At the instant when the X-ray dose reaches a predetermined emission stop threshold value (a target X-ray dose) the AEC function commands the X-ray source to stop X-ray emission. In the following description, both of the normal pixels and the measurement pixels are collectively called pixels. The pixel refers to an ingredient that has at least a conversion function for converting a small portion of the X-ray image into the electric charge.
The measurement pixel is the same as or several times larger than the normal pixel in size, and is disposed in one or a plurality of portions in the imaging area. Provided that the measurement pixel is the same size as the normal pixel, the normal pixel may be substituted with the measurement pixel or changed into the measurement pixel by easy modification. In some cases, the normal pixel may be used as the measurement pixel, or variation in a leak current or a bias current of the normal pixel may be detected to measure the X-ray dose therefrom. The small-sized measurement pixel does not hinder the detection of the X-ray image and hence facilitates detecting the X-ray image with high resolution, as compared with a conventional large-sized dose measurement sensor such as an ion chamber. Furthermore, selective use of the measurement pixels in accordance with a body part to be imaged makes it possible to precisely measure the X-ray dose passed through the body part.
By the way, in X-ray imaging, the X-rays produce scattered radiation in passing through the object. To remove this scattered radiation, a thin plate-shaped grid is used often. This grid is disposed between the object and the X-ray image detecting device, and preferably just in front of the X-ray image detecting device. There are two types of grids, one known as a movable grid swinging during X-ray imaging and the other known as a static grid standing still. In the following description, either type of grid is simply called grid except in cases where distinction between the types is necessary.
The grid is provided with strip-shaped X-ray transparent layers and X-ray absorbing layers that extend in a column direction of the pixels and are alternately and repeatedly arranged along a row direction of the pixels. Since the X-ray absorbing layer absorbs the X-rays passed through the object, widening the X-ray absorbing layers deteriorates the image quality of the X-ray image to be taken. Accordingly, the width of the X-ray absorbing layer is, for example, of the order of ⅕ to ⅓ of the width of the X-ray transparent layer, in general.
According to X-ray imaging using the grid, since the X-ray absorbing layers of the grid attenuate the X-rays to be incident upon the measurement pixels, a measurement value of each measurement pixel has to be calibrated to measure an X-ray irradiation amount (an X-ray exposure amount) of the object. This calibration method of the measurement value is described in US Patent Application Publication No. 2004/0096035 corresponding to Japanese Patent Laid-Open Publication No. 2004-166724, for example. First, X-ray imaging is performed in a state of disposing no object with and without using the grid. From two images obtained thereby, a correction coefficient of each individual measurement pixel is calculated such that an output signal of the measurement pixel (referred to as an AEC pixel in the US Patent Application Publication No. 2004/0096035) becomes the same between with and without the grid. In imaging using the grid, the output signal of the measurement pixel is multiplied by the correction coefficient to calibrate the X-ray dose.
An arrangement direction of the X-ray transparent layers and the X-ray absorbing layers of the grid is orthogonal to a row direction of the pixels. Provided that the normal pixel and the measurement pixel are of the same size, the size of one normal pixel (the pitch of the pixels) is 100 μm to 200 μm, and hence the size of the measurement pixel is of the order of 100 μm to 200 μm. On the other hand, there are two types of grids in which the number of the X-ray absorbing layers per unit length in the arrangement direction is 100/cm and 32/cm. By converting this number into a grid pitch (the arrangement pitch of the X-ray absorbing layers), grid pitches of 100 μm and approximately 300 μm are obtained.
Taking the case of a grid pitch of 300 μm and a measurement pixel size of 100 μm as an example, since the width of the X-ray absorbing layers is approximately 50 μm to 100 μm, a shift of the positional relation between the grid and the measurement pixels changes overlap between the measurement pixels and the X-ray absorbing layers and hence largely varies the output signals.
Since the X-ray transparent layers and the X-ray absorbing layers are regularly arranged at a constant period in the grid, an M or M+1 (M is an integer of 0 or more) number of X-ray absorbing layers are opposed to an arbitrary measurement pixel in accordance with the relation between the grid pitch and the size of the measurement pixels. Thus, in a case where the positional relation between the grid and the measurement pixels is shifted, variation in the output signal of the measurement pixel has its maximum value that corresponds to attenuation of the X-rays absorbed by one X-ray absorbing layer relative to the measurement pixel. Given that each X-ray absorbing layer has an almost constant X-ray absorptivity, the variation in the output signal is increased with decrease in the number M. If the grid pitch takes a value close to the size of the measurement pixels, the number M is a relatively small value. Therefore, the output signal of the measurement pixel is especially susceptible to the X-ray absorbing layer, and a problem of measurement precision of the X-ray dose owing to the shift of the positional relation between the grid and the measurement pixels becomes conspicuous.
The effect of the X-ray absorbing layers can be calculated from an image of the stopped movable grid or the still grid captured in the absence of the object. According to experiment of the inventors, it is apparent that a pixel value is decreased on the order of 20% by a certain grid owing to the effect of the X-ray absorbing layer, by comparison between a large pixel value and a nearby decreased pixel value that the X-ray absorbing layer affects.
In a case where the grid is secured to the X-ray image detecting device, variations in attachment position of each part in manufacturing cause the shift of the positional relation between the grid and the measurement pixels. In the case of an electronic cassette separate from the grid or in the case of the grid detachable from an imaging stand or an imaging table, variations in loading position of the electronic cassette or the grid cause the shift of the positional relation between the grid and the measurement pixels. In some cases, the positional relation between the grid and the measurement pixels may be shifted by vibration and the like while imaging is repeatedly performed.
In the calibration method of the measurement pixels according to the US Patent Application Publication No. 2004/0096035, in a case where the positional relation between the grid and the measurement pixels is shifted whenever imaging is performed, a lot of calibration images are prepared in accordance with a shift amount, and the shift amount is detected in the imaging on the order of μm corresponding to the grid pitch. One of the calibration images is chosen in accordance with the shift amount, and the correction coefficient to correct sensitivity of the measurement pixel is calculated. This calibration method of the measurement pixels precisely measures the shift amount and allows calibration with high precision, but requires the many calibration images. Also, in a case where the positional relation between the grid and the measurement pixels is shifted during manufacturing, the calibration images have to be taken on a product-by-product basis, and its preparation operation requires much time and effort. Furthermore, a huge number of calibration images have to be prepared at the thought of oblique incidence of the X-rays upon the imaging area, so that realization is difficult. Accordingly, it is desired that the X-ray dose can be measured easily and precisely even if the positional relation between the measurement pixels (the dose measurement sensors, in general) and the grid is shifted, without using the huge number of calibration images.